short answer:
The same way as with 3-ply.
You start with an overhand knot of the 2 plies and continue with passing over under similarly to the  3-ply method. The result will look like 4 strand braid.


I have tried to backsplice into the backsplice and it can be done but the release knot gets too heavy and bulky.
https://www.youtube.com/watch?v=8hObxtOx-GY look at the end of the video to see how they look like